Remote communication utilizing wireless equipment typically relies on radio frequency (RF) technology, which is employed in many industries. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles.
RF identification (RFID) tag systems have been developed to identify, monitor, or control remote objects. As shown in FIG. 1, a basic RFID system 10 includes an interrogator 18 and transponders (commonly called RF tags) 16. The interrogator 18 includes a transceiver with decoder 14 and an antenna 12. The tag 16 includes an antenna 24. In operation, the antenna 12 emits and receives electromagnetic radio signals generated by the transceiver 14 to activate the tag 16, and receive signals from the tag. When the tag 16 is activated, data can be read from or written to the tag.
In some applications, the transceiver and antenna 12 are components of an interrogator (or reader) 18, which can be configured either as a hand-held or a fixed-mount device. The interrogator 18 emits the radio signals 20 in range from one inch to one hundred feet or more, depending upon its power output, the radio frequency used, and other radio frequency considerations. When an RF tag 16 passes through the electromagnetic radio waves 20, the tag detects the signal 20 and is activated. Data encoded in the tag 16 is then transmitted by a modulated data signal 22 through an antenna 24 to the interrogator 18 for subsequent processing.
An advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. Tags can be read through a variety of substances such as snow, fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RF tags can also be read at remarkable speeds, in most cases responding in less than one hundred milliseconds.
There are three main categories of RFID tag systems. These are systems that employ beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in fundamentally different ways. The invention described below in the Detailed Description can be embodied in any of these types of systems.
The beam-powered RFID tag is often referred to as a passive device because it derives the energy needed for its operation from the radio frequency energy beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity (RF cross-section) that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to change its reflectivity that is seen at the interrogator to develop a communication link. Here, the battery is the only source of the tag's operational power. Finally, in the active RFID tag, both the tag and reader have transceivers to communicate and are powered by a battery.
A typical RF tag system 10 will contain at least one tag 16 and one interrogator 18. The range of communication for such tags varies according to the transmission power of the interrogator 18 and the tag 16. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach in excess of 100 meters in range, depending on the frequency and environmental characteristics.
Conventional RF tag systems utilize continuous wave backscatter to communicate data from the tag 16 to the interrogator 18. More specifically, the interrogator 18 transmits a continuous-wave radio signal to the tag 16, which modulates the signal 20 using modulated backscattering wherein the electrical characteristics of the antenna 24 are altered by a modulating signal from the tag that reflects a modulated signal 22 back to the interrogator 18. The modulated signal 22 is encoded with information from the tag 16. The interrogator 18 then demodulates the modulated signal 22 and decodes the information.
Conventional continuous wave backscatter RF tag systems utilizing passive (no battery) RF tags require adequate power from the signal 20 to power the internal circuitry in the tag 16 used to modulate the signal back to the interrogator 18. While this is successful for tags that are located in close proximity to an interrogator, for example less than three meters, this may be insufficient range for some applications, for example greater than 100 meters.
A problem in RFID systems is in the rapid identification of an unknown number and identity of tags with long IDs in the field of view of the reader.